An ultrasonic motor in which an annular and disc-like rotor is brought into pressure-contact with the surface of an oscillator (stator) including an annular or disc-like bimorph which is excited to generate a travelling flexing wave so that the rotor is rotated has recently been proposed and some type of such ultrasonic motors have been already put into market.
However, it is very hard to form an ultrasonic motor which is high in efficiency and can be stably rotated and generates no audible noise. That is, in order to efficiently convert the oscillating energy generated by the oscillator which is a driving source of the ultrasonic motor into a rotational movement of a rotor, the following requirements should be satisfied.
Firstly, leakage of the vibration generated at the stator through a support of the vibrator to the outside should be prevented. However, this problem can be solved by adopting a supporting structure disclosed in the Japanese Patent Application Sho/63-50224 filed by the present assignee.
Secondly, it is necessary to provide a rotor structure in which the loss of the vibration energy of a stator due to generation of a vibration mode which will not contribute to the rotation of the rotor is prevented.
Thirdly, it is necessary to form such a motor in such a way that the vibration component of the vibration generated at the stator in a rotor driving direction is efficiently transmitted to the rotor and the vibration components in a direction normal to the rotor driving direction will not be impeded. Japanese Unexamined Patent Publication Sho/63-174581 discloses a rotor structure which satisfies the second and third requirements. As shown in FIG. 14, in detail, the width of a contact portion 3 of a rotor 1 which is in contact with a main body of a stator 4 is made as narrow as possible in this rotor structure. A portion which causes a contact portion 3 of the rotor 1 to pressure-contact with the stator 4 comprises a thin-walled flange 2.
However, unless the radial width of the contact portion 3 is as narrow as possible and the thickness of the contact portion 3 is substantially as thin as the flange 2 in the prior structure as shown in FIG. 14, the flexing vibration causes the contact portion to generate a vibration which will not contribute to the rotation of the rotor. The vibration will not be dampened at the contact portion 3 and the flange portion 2. The vibration of the stator main body 4 is transmitted to the rotor 1 so that the vibration of the stator main body 4 is leaked to the outside via the rotor 1. Since a true contact area between the contact portion 3 and the rotor surface on driving of the ultrasonic motor is proportional to the width of the contact portion 3, a sufficiently large contact area can not be formed. Accordingly a large load is locally imposed upon the contact area between the contact portion 3 and the rotor, resulting in a remarkable wear at the contact area. Therefore, the lifetime of the ultrasonic motor is short even when tungsten carbide or hard alumite etc. having an excellent wear resistance is used for the contact surface.
Furthermore, it is hard to machine the thin-walled contact portion 3 at a high precision. As the flatness of the contact portion is insufficient, the ultrasonic motor will not rotate in a stable manner and in an extreme case, it will not rotate at all.